Gas Chromatography (GC) is one of the most important tools used in chemical analysis Recently, GC has been advanced by the development of fused silica bonded stationary phase capillary columns. These columns can be used to generate chromatograms showing both high speed of analysis and high resolving power relative to the previously used packed GC columns. In addition, these columns are much less likely to be broken during normal handling than ordinary glass capillary columns.
Many sample injection techniques are used in GC such as split, splitless, flash and on-column injection. The on-column technique has several advantages and is characterized by the injection of a liquid sample directly into the inlet of a GC column. A fused silica bonded stationary phase capillary column is ideal for on-column injection because the stationary phase is chemically bonded to the interior surface of the capillary column and can not be washed off by the liquid sample. On-column injections into fused silica bonded stationary phase capillary columns using syringes having long, fine stainless steel or fused silica needles are routinely made. However, this on-column technique is not easily automated and the needles are frequently damaged in use. When very large on-column injections are made, e.g., greater than 5 microliters, the fused silica bonded stationary phase capillary column is generally preceded by a retention gap which is simply a section of fused silica capillary tubing that is usually deactivated but not coated with a stationary phase and the injection is made into the retention gap.
Steele and Vassilaros, Journal of High Resolution Chromatography & Chromatography Communications. Vol. 6, 1983, pp. 561-563, advanced the art of on-column injection with fused silica bonded stationary phase capillary columns by injecting a predetermined volume (1 microliter) of a sample with a loop-type rotary injection valve. A problem with the system of Steele and Vassilaros was sample carryover, i.e., not all of the sample is moved from the injection valve into the column when the valve is rotated to the inject position and the portion that remains tends to contaminate the next injection. Another problem with the system of Steele and Vassilaros occurs when the volume of sample is very large, e.g., more than 5 microliters to as much as several hundred microliters, and a retention gap is used. This problem is the long length of time needed for the solvent peak to elute.
Hopper, Journal of Chromatography, Vol. 302, 1984, pp. 205-219, used a rotary ten-port valve for on-column injection using both fused silica capillary columns and packed columns. The ten-port valve of Hopper incorporated a wash loop (1 microliter) in addition to a sample loop (2 microliters) so that solvent in the wash loop followed the sample onto the column to reduce the problem of sample carryover. Although the valve of Hopper does help solve the problem of sample carryover, it does so with an increased complexity of the system, i.e., a more complex valve and the use of the wash solvent. In addition, the system of Hopper when used for very large injections with a retention gap still suffers from the long length of time needed for the solvent peak to elute. Nevertheless, the rotary injection valves of Steele et al. and Hopper eliminated the use of a fragile syringe needle and made it possible to automate the injections.